1. Field of the Invention
This invention relates to molding compositions and forming processes for normally rust-prone iron-based powders, and articles produced therefrom. Metal alloy systems that can be successfully formed using the processes of the invention, include elemental iron and iron alloys including low and medium alloy steels, tool steels and a number of specialty iron-base alloys.
2. Description of the Related Art
The prototypical process for forming metal powders is Metal Injection Molding (MIM). The steps of fabrication of metal or ceramic-metallic (CERMET) parts are the following:
i. Metal and/or ceramic powders are blended with a thermoplastic binder material and shredded or pelletized to create an injection molding feedstock with thermoplastic properties.
ii. The thermoplastic feedstock is injection molded in a fluid state using methods and tools typical of conventional plastic injection molding, and removed from the mold in a solid state.
iii. The xe2x80x9cgreenxe2x80x9d state as-molded parts are subjected to thermal and/or chemical processes to remove the binder phase.
iv. The remaining xe2x80x9cbrownxe2x80x9d state metal or CERMET parts are sintered at higher temperatures to effect consolidation and densification of the molded object.
Several methods, processes, and binder systems have previously been described for fabrication of rust prone iron-based metal alloys and CERMET materials containing them. Each of these processes has one or more disadvantages that prevents important applications.
For example, commonly utilized polymer or wax binder MIM processes, such as the methods described by Achikita et al. in U.S. Pat. No. 5,250,254, are limited to small parts, weighing no more than a few hundred grams, and with maximum section thickness of less than 10 millimeters. This limitation is imposed by the difficulties associated with binder removal prior to sintering. The manufacture of larger parts is prevented or rendered uneconomical by dimensional instability, cracking, or simply the long times needed for binder removal from larger and thicker sections. In addition, great care must by taken when using wax or resin binders to avoid an undesirable out-of-specification increase in the carbon content of the alloy as a result of incomplete removal of the hydrocarbon binder phase.
Fanelli et al., in U.S. Pat. No. 4,734,237, disclose agaroid-based aqueous binders for molding of metal and ceramic powders. The development of aqueous-binder molding compositions, including those disclosed by Fanelli et al., has largely removed the part size restrictions imposed by wax and polymer binders since the binder phase in these largely consists of water which is easily removed by evaporation under ambient conditions. In the special case of agar-based binders, the carbon content problem associated with wax and polymer binders is also effectively addressed since the agar component of the binder is largely gasified at relatively low temperatures during the early stages of the sintering cycle. Further reduction in carbon content is easily achieved by employing an oxidizing atmosphere in the early stages of the sintering heat treatment as taught by Zedalis in U.S. Pat. No. 5,985,208. Carbon content can also be reduced by heat treatment in hydrogen.
Borate and polyamine additives to enhance the gel strength of agar-based aqueous binders have been disclosed by Sekido et al. in U.S. Pat. No. 5,258,155, Rohrbach et al. in U.S. Pat. No. 5,286,767 and Fanelli et al. in U.S. Pat. No. 5,746,957.
Zedalis et al., in U.S. Pat. No. 6,268,412, incorporated herein by reference to the extent not incompatible herewith, disclose molding compositions and processing steps for injection molding of non-rust-prone stainless steel articles using water-base agaroid binder systems. Stainless steels, a family of iron-based alloys containing between 10.5 and 28 atomic % chromium, are compatible with water-based binder systems, since the high chromium content confers great resistance to oxidation in the presence of water.
When rust-prone iron-base alloy powders are substituted for the stainless steel powders in the process taught by Zedalis, the resulting molding feedstock is chemically unstable and must be molded and dried within hours or days, or the water will react with the iron-base alloy powder to form rust, thereby substantially altering and degrading the rheological properties, sintering, and shrinkage behavior of the feedstock.
It is commonly observed that ferrous alloys progressively oxidize or rust in the presence of air and moisture. The essential chemistry of rust formation, as described in The Metals Handbook, Volume1, 8th Edition, published by the American Society for Metals, (1961), p257, follows. In the first step of the reaction, iron reacts with water to form ferrous and hydroxyl ions and hydrogen:
Fe+2H2O=Fe+++2OHxe2x88x92+H2xe2x80x83xe2x80x83(1)
In a second step, oxygen, if present, reacts with the ferrous ions to produce ferric ions which precipitate out of solution as insoluble ferric hydroxide FeO(OH), otherwise known as rust. Since the rust deposit does not form a protective layer, reaction 1 is free to proceed until the metallic iron is consumed or equilibrium is reached.
The equilibrium constant for reaction 1 is:
K=[Fe++][OHxe2x88x92]2PH2xe2x80x83xe2x80x83(2)
where the square brackets indicate the concentration of the species and PH2 is the partial pressure of hydrogen.
Equation 2 suggests that the equilibrium concentration of Fe++can be suppressed by increasing the hydroxyl ion concentration, equivalent to increasing the pH, and or increasing the hydrogen partial pressure. Numerous hydroxyl ion sources, including alkali metal hydroxides and carbonates, ammonia, and various organic amines have been used to inhibit rusting of ferrous alloys in applications involving intermittent or continuous exposure to water.
Rusting can be further inhibited by passivation of the exposed ferrous alloy surface. Typically, passivation involves a thin but impervious layer of gamma iron oxide formed, in-situ, by reaction of the iron with oxidizing ions. Pourbaix, in Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, New York P. 312 (1966), states that passivation of iron is difficult at a pH below 8, relatively easy at a pH above 8 and very easy between pH 10 and 12. Above pH 13, however, iron will corrode by hyperferrate ion formation. Passivation of ferrous alloy surfaces can be accelerated by the deliberate addition of oxidizers to aqueous environments. Nitrite and nitrate salts have been used in this manner as rust-inhibiting additives in cooling water and other process water applications pH buffers, salts formed by reaction of weak acids with strong bases, are frequently employed with nitrite and nitrates to maintain pH in the proper range. The Metals Handbook, Vol.1, 8th Ed., American Society for Metals, P. 279, 1961 states that sodium nitrate-borate combinations have been used to inhibit corrosion in diesel engine cooling systems and in low pressure, hot water recirculating systems. In this case, sodium borate (formed by reaction of the weak acid H3BO3 with the strong base NaOH), functions as a pH buffer. In a similar fashion, calcium nitrite is frequently added to concrete formulations to inhibit rusting of embedded steel reinforcing bars. In this case, the required alkaline environment is synergistically provided by the calcium oxide component of the Portland cement concrete.
Behi et al. in U.S. Pat. No. 6,261,336, specifically addressed the problem of rust formation in aqueous injection molding feedstocks containing rust-prone ferrous alloy powders, and taught that these materials can be stabilized against rust formation by the addition of alkaline sodium silicate to the aqueous binder. It was shown by Behi that carbonyl iron powder feedstocks containing appropriate amounts of sodium silicate are somewhat stable against rust formation and attendant hydrogen evolution, and that the stability is further enhanced by the addition of potassium borate. While this was an improvement over unstabilized iron-based aqueous binder feedstocks, experience with the sodium silicate stabilized feedstocks has revealed that the sodium silicate addition unfortunately renders the feedstock pellets tacky and difficult to feed into the barrel of an injection molding machine. Moreover, residual SiO2 and/or iron silicate inclusions, resulting from decomposition of higher loadings of the sodium silicate during sintering, may be undesirable for applications requiring maximum ductility and fatigue resistance in the final sintered steel part.
A need exists for methods enabling molding of rust prone iron-based alloys that avoid the size limitations of the prior art wax and polymer based binders and the processing and ductility limitations of sodium silicate stabilized aqueous binders.
In one embodiment, the invention is a molding composition comprising at least one metal powder selected from the group consisting of elemental iron, an iron-base alloy containing less than 10 wt. % chromium, and an iron-based intermetallic compound; and a binder comprising a gel forming material, a solvent for said gel forming material; and at least one compound selected from the group consisting of inorganic nitrates and inorganic nitrites.
In another embodiment, the invention is a process comprising the steps of: injecting an aforedescribed molding composition into a mold at a temperature above the gel point of said gel-forming material; cooling said molding composition in the mold to a temperature below the gel point of said gel-forming material to produce a self supporting molded article; removing said article from the mold; substantially removing said solvent from said molded article; and sintering said molded article in a protective atmosphere and under such conditions of time and temperature as are required to produce a final density greater than about 90% of the theoretical density.
In another embodiment, the invention is a process comprising the steps of: feeding an aforedescribed molding composition into an extruder; extruding said molding composition through a shape forming die; cooling at least the surface of said extrudate to a temperature below the gel point of said gel-forming material to produce a shaped article with at least a self supporting skin; substantially removing said solvent from said shaped article; and sintering said shaped article in a protective atmosphere and under such conditions of time and temperature as are required to produce a final density greater than about 90% of the theoretical density.
In yet another embodiment, the invention comprises shaped articles comprising a metal powder selected from the group consisting of elemental iron, an iron-base alloy containing less than 10 wt. % chromium, and an iron-based intermetallic compound produced by one of the aforedescribed processes.